Field of the Invention
The present invention relates to a circuit for a DC fan and in particular to a power-off brake circuit for a DC fan, which achieves the effect of power-off brake by means of an independent brake loop and effectively improves the accurate control of rotating speed.
Description of Prior Art
In many customer products, the fan is a main heat-dissipation tool. When the temperature of a system is too high, the fan will operate to reduce the temperature of the whole system to achieve the effect of heat dissipation. When the temperature of the system decreases to a specific value, the fan will stop operating.
In the traditional design, to prevent the whole system from burning up due to excessive temperature, the system is usually equipped with a cooling fan for protection. However, the requirement of rotating speed of the fan is getting stricter to effectively dissipate the heat generated by the increasing complex devices. Thus, after the fan powers off, the fan still keeps rotating for a period of time and then stops totally.
As for the traditional power-off brake technology for the DC fan, it uses an H-bridge of the motor driver circuit in a capacitive energy storage device such that when power-off occurs, two PMOS transistors on upper arm of the H-bridge or two NMOS transistors on the lower arm of the H-bridge are conducted to make two ends of the motor coil connected to the H-bridge be shorted, resulting in no change in the magnetic poles of the motor and thus the braking effect. Though the fan can be braked using the above traditional technology, it incurs another problem which is that the MOS transistors (e.g., NMOS transistors or PMOS transistors) used in the actuation of power-off brake in the fan are common to those (i.e., two PMOS transistor on the upper arm or two NMOS transistors on the lower arm) used in the H-bridge of the motor driver circuit in the fan. For the traditional fan to control the rotating speed or brake, it needs to use a front driver to control the actuation of the PMOS transistors on the upper arm of the H-bridge and the NMOS transistors on the lower arm of the H-bridge, but such a control method will cause signal distortion during the PMOS transistors in PWM (pulse width modulation) width chopping process and fail to achieve accurate control of rotating speed. Also, the signal distortion can cause a low-voltage-turn-on problem regarding MOS transistors and generate waste heat, which results in a temperature increase in the devices.
Therefore, how to overcome the above problems and disadvantages is the focus which the inventor and the related manufacturers in this industry have been devoting themselves to.